This is what fixed it for me.

If you're using a messaging service, which you have to now for A2P, then the FROM variable in the REST call should be the Messaging SID, not a phone number...

This solution is burried in the documentation and not mentioned anywhere in tutorials...

https://help.twilio.com/articles/17894300978843-FAQ-Why-Does-the-Send-Wait-for-Reply-Widget-Stop-Collecting-the-Users-Response-and-Creates-a-New-Execution-for-US-A2P-Registered-10DLC-Numbers